Increasingly greater attention is being paid to the protection of materials such as metals, ceramics and carbonaceous substances which can be employed especially at higher temperatures. Such materials, mainly metals and CFC (carbon fiber-reinforced carbon), are coated with ceramic coatings, for example SiC coatings, especially as protection against oxidation. Because of the very different coefficients of expansion (substrate/ceramic), exposure to heat generally gives rise to microcracks in the protective coating through which substances causing corrosion of the material, such as atmospheric oxygen, acids etc., are able to penetrate. This problem can be partially alleviated by using protective coatings made of glass.
The production of vitreous protective coatings on materials made of metal, ceramics or carbonaceous substances is already known. In British Patent 1,134,756 (Battelle Memorial Institute), a process for the production of a vitreous protective coating is described in which a solution of polyborosiloxanes in an inert solvent is applied to the surface to be protected. After removal of the solvent the coating is subjected to heat treatment. This process has the disadvantage that it is always necessary to use a solvent and the resultant glass coatings do not have adequate mechanical stability and low resistance to changes in temperature. German Patent 2,930,557 (Yajima, Seihi; The Research Institute for Special Inorganic Materials) and the corresponding U.S. Pat. Nos. 4,267,210 and 4,267,211 describe a coating composition which is resistant to corrosion, heat and oxidation, in which the composition consists of an organoborosiloxane compound which has been modified with aliphatic and aromatic alcohols, phenols and aromatic carboxylic acids or with transition metals, metals of the actinium group, metals of the lanthanum group, alloys of these metals, persulfate salts, organic peroxides and halides of Al, Fe, B, Ga and In. The modified organoborosiloxane compound, must be prepared in an atmosphere which is inert or non-oxidizing to the reaction, at a high temperature, then dissolved in an organic solvent, and applied to the substrate to be protected and then heat-treated.
It is an object of the present invention to provide vitreous coating compositions. Another object of the present invention is to provide a process for preparing vitreous coatings having high flexibility. A further object of the present invention is to provide vitreous coatings which are highly flexible, have high stability to chemical and physical influences, are highly resistant to changes in temperature, and exhibit good adhesion to substrates as well as provide good sealing of the substrates.